I don't think impact alone was the cause, though it was a factor.
The entire pistol was drawn by an extremely powerful magnetic field. When the pistol made contact it stopped but the firing pin was still drawn by the field, that magnetic attaction with the addition of inertia of the mass flying through the air allowed the pin to strike with much greater force than if simply dropped or even thrown at the same velocity.

The Magnetic field was obviously much stronger than the gravitional field at this localized point in space, otherwise the pistol would have dropped to the floor rather than flying near parallel to the surface of the earth.